EL Resolvers
EL Resolvers
EL Resolvers
At the center of the EL machinery is the extensible ELResolver class. A class that implements
ELResolver
defines how to resolve expressions referring to a particular type of object or
property. In terms of the following expression, a BeanELResolver instance is called the first
time to find the base object, employee, which is a JavaBeans component. Once the resolver finds
the object, it is called again to resolve the property, lName of the employee object.
${employee.lName}
The unified EL includes a set of standard resolver implementations.
lists these
standard resolvers and includes example expressions that they can resolve.
TABLE 54
Standard EL Resolvers
Resolver
Example Expression
Description
ArrayELResolver
${myArray[1]}
Returns the value at index 1 in the array called
myArray
BeanELResolver
${employee.lName}
Returns the value of the lName property of the
employee
bean
ListELResolver
${myList[5]}
Returns the value at index 5 of myList list
MapELResolver
${myMap.someKey}
Returns the value stored at the key, someKey, in
the Map, myMap
ResourceBundleELResolver
${myRB.myKey}
Returns the message at myKey in the resource
bundle called myRB
Depending on the technology using the unified EL, other resolvers might be available. In
addition, application developers can add their own implementations of ELResolver to support
resolution of expressions not already supported by the unified EL by registering them with an
application.
All of the standard and custom resolvers available to a particular application are collected in a
chain in a particular order. This chain of resolvers is represented by a CompositeELResolver
instance. When an expression is encountered, the CompositeELResolver instance iterates over
the list of resolvers and consults each resolver until it finds one that can handle the expression.
If an application is using JSP technology, the chain of resolvers includes the
ImplicitObjectELResolver
and the ScopedAttributeELResolver. These are described in the
following section.
See section JSP 2.9 of the JavaServer Pages 2.1 specification to find out the order in which
resolvers are chained together in a CompositeELResolver instance.
To learn how to create a custom EL resolver, see
.
Unified Expression Language
Chapter 5 · JavaServer Pages Technology
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